Collective spin in twisted bilayer materials

Twisting has been demonstrated as a technique for creating strongly correlated effects in two-dimensional bilayered materials and can tunably generate nontrivial topological properties, magnetism, and superconductivity. Magnetism is particularly significant, as it can both compete with superconductivity and lead to the emergence of nontrivial topological states. Due to the technique challenges in theoretical methods without input parameters, theorical studies on twisted materials beyond twisted bilayer graphene are rarely reported. Thus, first-principles studies on a wide range of materials are expected in both fields of fundamental physics and materials science. By using self-developed large-scale density functional theory calculations uniquely capable of simulating twisted systems, we found the ferromagnetism arising due to spin splitting in different twisted bilayer systems across various two-dimensional materials, such as twisted bilayer $h$-BN, $2H\ensuremath{-}\mathrm{MoT}{\mathrm{e}}_{2}$, PbS, and $2H\ensuremath{-}\mathrm{NbS}{\mathrm{e}}_{2}$. The spin splitting itself is induced by the enhanced ratio of the exchange interaction to band dispersion near the Fermi level. An important discovery in this paper is that, in some nonmagnetic metallic bilayers, e.g., $2H\ensuremath{-}\mathrm{NbS}{\mathrm{e}}_{2}$ and $1T\ensuremath{-}\mathrm{Mo}{\mathrm{S}}_{2}$, twisting transforms a nonmagnetic state into a collective ferromagnetic state even without any doping.